the thalamus and cerebral cortex collectively. These parts of the brain are so closely and reciprocally interconnected—especially in mammals—that they are often treated as a single system. Normal functioning of this system appears to be necessary for conscious experiences and voluntary actions. During waking states, groups of neurons in this system exhibit a pattern of continuous firing. During states of reduced consciousness, such as deep sleep, the entire system goes into an oscillatory mode characterized by neuronal burst–pause firing.